Automatic powershiftable transmissions for vehicles, known as automatic transmissions for short, usually have as the starting element a hydrodynamic torque converter. Predominantly, these transmissions are designed as range-change transmissions which comprise several planetary gearsets for obtaining a number of gears or gear steps, which are usually engaged by means of hydraulic shifting elements such as disk clutches or disk brakes.
The hydrodynamic torque converter consists of an oil-filled housing, in which a pump impeller is connected as the drive input to a crankshaft of an internal combustion engine and a turbine wheel is connected as the drive output to a transmission input shaft. In addition, between the pump impeller and the turbine wheel, a guide wheel that can move in the direction of a freewheel is provided as a supporting element for the torque conversion. During operation the pump impeller, driven by the combustion engine of the vehicle, sets the converter oil into motion. The kinetic flow energy of the oil is taken up by the turbine wheel and converted to rotational movement which is transmitted to the transmission. The guide wheel deflects the oil flowing back from the turbine wheel, so that the torque of the turbine wheel delivered to the transmission is larger than the torque of the pump impeller taken up by the combustion engine. However, since the pump impeller always runs ahead of the turbine wheel, there exists a slip that depends on a speed difference and this reduces the efficiency of the converter. Accordingly the hydrodynamic converter is mostly used only as a starting element in combination with a range-change transmission or a continuously variable transmission, and also comprises a bridging clutch which bridges across the pump impeller and the turbine wheel by friction after the starting process.
For its proper function the transmission needs an effective oil supply for lubrication and cooling, as well as particular filling quantities and oil pressures for switching the hydraulic components. For this, as a rule a hydraulic pump coupled to a transmission input shaft and driven by the combustion engine is provided, which delivers the necessary oil pressure by way of a regulated, valve-controlled hydraulic system.
Furthermore, to reduce the emission of harmful substances, fuel consumption and noise levels, particularly in urban traffic, it is desirable to provide a so-termed start-stop function in which, depending on the situation and possibilities, the combustion engine should be switched off when the vehicle is at rest, for example at road junctions with traffic lights. After a subsequent engine start the transmission must be ready to transmit torque again as soon as possible. However, in vehicles with converter automatic transmissions this is not straightforward.
Since the hydraulic pump is driven by the combustion engine, the pressure oil is only available when the combustion engine is running. In contrast, when the combustion engine is switched off the hydraulic system loses pressure, whereupon the transmission control system usually disengages the current gear and shifts the transmission to neutral. Before torque can be transmitted again, a certain pressure has to build up in the hydraulic system again before the vehicle can start off. This results in a relatively long activation time until the gear is engaged after the engine is started.
On the other hand, if the transmission of torque were to begin too soon, i.e. before a sufficient pressure has built up in the piston chambers, the corresponding frictional bodies would undergo slip. Since if the oil pressure is insufficient the cooling and lubrication of wet-operating shifting elements (disk brakes) is not ensured, so that in some circumstances high clutch loads and wear result, the shifting elements concerned could be damaged within a very short time.
To prevent that, a safety interval must be maintained, within which in every case a sufficient pressure has built up before a selected gear can be engaged. However, this safety interval further prolongs the activation time and that makes it practically impossible to have start-stop operation with a high frequency of starting processes and the need, after the engine has started, for example when the traffic lights change, to start off as promptly as possible since in practice there will be frequent and long delays.
To circumvent or shorten the safety interval a special pressure-sensing system with pressure sensors in the transmission could be provided, which explicitly indicates the pressure in the pressure circuit of the hydraulic system or in the piston chambers of the disk clutches and passes that information on to the transmission control system. However, that is relatively complicated, it would incur higher manufacturing costs and would require additional components.
DE 10 2007 003 924 A1 discloses a method for controlling an automatic transmission of a vehicle. To check the torque-transmitting capacity of the automatic transmission, when the internal combustion engine is started a time variation of the rotational speed of the turbine wheel of a starting element is determined. With the help of known rotational speed characteristic of the turbine wheel, an activation time-point is then determined, at which a torque-transmitting capacity of the automatic transmission is obtained by virtue of a sufficient build-up of cooling and pressure oil. A disadvantage of this method is that to carry out the method the automatic transmission has to be operated on a test bed with the transmission output free, i.e. with the shifting elements still open.